This invention relates to reciprocatory piston and cylinder machines. It is particularly applicable to vacuum pumps of the piston and cylinder type which can operate without use of a lubricating or sealing liquid and which can be used as backing pumps for electron microscopes.
At present almost all electron microscopes are equipped with pumping systems based on oil diffusion pumps backed by oil-filled rotary mechanical pumps of the vane type. Consequently, a considerable part of the residual gas in the columns of these instruments is contributed by molecules of oil and fragments of oil molecules, so a contaminating layer of carbonaceous material is deposited on the specimen and on all surfaces irradiated by the electron beam. The contamination of the specimen can be substantially reduced by surrounding it with a liquid nitrogen cooled trap and this practice is widely adopted.
The problem of contamination in electron microscopes could best be avoided by the use of oil-free pumps. Most previous attempts to produce oil-free pumps have involved modifications of the rotary type pumps and have been unsuccessful but Australian Pat. No. 481,072 does disclose a pump of the reciprocating piston and cylinder type which is capable of producing high vacuum conditions without the use of lubricating and sealing oil. However, the vacuum which can be achieved with a piston and cylinder type of pump operating under oil-free conditions is limited by difficulties in sealing against gas leakage into the working spaces of the pump and, in conventional constructions, by the need to have valves which need to be subjected to gas pressure to open. The vacuum that can be produced in the high vacuum stage of a multi-stage pump can then be determined by the pressure required to open an exhaust valve in that stage of the pump. The present invention enables alleviation of these difficulties.